1. Field of the Invention
The present invention relates to a method of heat treating an object to be processed such as a semiconductor wafer.
2. Description of the Related Art
In the conventional steps in manufacturing a semiconductor device, a heat-treating apparatus comprising a cylindrical process tube is used. The heat-treating apparatus performs predetermined heat treating to a semiconductor wafer serving as an object to be processed at a predetermined pressure and a predetermined temperature in the process tube. In recent years, as a heat-treating apparatus, a vertical type apparatus in which a process tube is vertically arranged has been popularly used in place of a conventional horizontal type apparatus in which a process tube is horizontally arranged.
In the vertical heat-treating apparatus, the process tube is formed of quarts or fused silica, the lower end of the process tube is open, and a semiconductor wafer serving as an object to be processed is loaded/unloaded in/from the lower opening. In this case, a plurality of wafers are placed on a wafer boat such that the surfaces of the wafers are kept horizontal, the wafers in this state are loaded/unloaded in/from the process tube. A heater and a heat insulator are arranged around the process tube to heat interior of the process tube, and a discharge pipe and a process gas supply pipe are connected to the lower portion of the process tube.
In the heat-treating apparatus, the wafer boat with the wafers is loaded in the process tube, the process tube is evacuated from the discharge pipe, and a predetermined process gas is supplied from the process gas supply pipe. In this state, a predetermined process, e.g., a CVD process, is performed to the semiconductor wafers at a predetermined process temperature and a predetermined process pressure.
In recent years, semiconductor devices tend to be highly integrated, and circuit patterns of the semiconductor devices tend to be further micropatterned accordingly. For this reason, in each processing step in the manufacture of a semiconductor device, the accuracy of each processing step must be improved. In heat treating, a heat-treated layer rarely containing an impurity must be obtained by improving the processing accuracy. For this purpose, improvement of the purity of a process gas and an increase in degree of vacuum in heat treating are considered.
However, even when the purity of the process gas is improved, and the heat treating is performed at a predetermined degree of vacuum, a gas is produced from semiconductor wafers and the inner wall of a process tube during the heat treating. For this reason, a small amount of impurity mixes in the atmosphere, thereby degrading the purity of a heat-treated layer.
In addition, improvement of processing accuracy easily causes a considerable decrease in throughput, and high-purity annealed layers must be formed while the decrease in throughput is suppressed.